PC Upgrade

Hello guys, I've few questions for you.

Now, I'm running on:

Gigabye GA-EP35-DS3L
C2D E6420 2.13@3.2
2x 2Gb OCZ Reaper 1066 Mhz
EVGA GF 8800GTS 320MB SuperClocked
2x HDD Seagate 320 i 500Gb
LC Power 550W

I want to buy:

Corsair CMPSU-520HXEU 520W or OCZ ModXStream 500W
Gainward GTX260 896MB 448bit PCI-E DDR3 Golden Sample 55nm or PowerColor HD4870 512MB GDDR5

I'm running on 1280x1024.
Now the questions:
Is there any sense to change my CPU? I think about E8400, but doesn't the 3.2Ghz will be enough for me?
And what do you think about these parts? Is it good? And what shall I chose: OCZ or Corsair, Nvidia or Ati, or something else?

Sorry for my english and thanks in advance,
Patrick.

I think I could easy OC E8400 to 4GHz.

Yes, you could do that pretty easy but anything over 3.6 you really don't see any benefit gaming wise.

JonnyGuru review of 550VX
It actually got a higher rating than the 520HX
GTX 275 or HD 4890 - close enough not to really matter. Since you have a CF MB it makes sense to go with 4890. As long as you get a decent price the Sapphire 4890 Vapor-X is a good choice.

You'll probably save enough going with the Corsair 550VX over the 520HX to pay for the 4890 upgrade over the 4870.
The CPU upgrade might have got you a few extra FPS but they'd have been VERY expensive extra FPS considering the cost of the upgrade.